Currently, the most common forms of treating breast cancer involve surgery, chemical intervention, and/or radiotherapy. Unless the cancer is restricted to a defined area, surgery alone cannot eliminate the cancer. Accordingly, radiation treatment is often given after surgery to destroy cancer cells that are near the surgical site and that have evaded surgery. The side effects of such treatment include skin sensitivity or itchiness, interference with the immune system, sometimes queasiness and, rarely, radiation fibrosis where an affected portion of the lung becomes fibrous. Chemotherapy may also be employed following surgery. Chemotherapy utilizes drugs that are toxic to cancer cells. Since this is not a perfectly selective system, normal cells are affected as well. Negative side effects include nausea, tiredness, loss of appetite, hair loss and diarrhea.
In view of the disadvantages of the present therapies, attempts have been made to find additional approaches for treating breast cancer. One such approach is immunotherapy. One of the targets for an immunotherapeutic approach is the HER-2 protein. The HER-2 protein, a product of the HER-2 oncogene, is overexpressed in a variety of cancers. It is found in 50%–60% of ductal in situ carcinoma and 20%–40% of all breast cancers, as well as a substantial fraction of adenocarcinomas arising in the ovaries, prostate, colon and lung. Overexpression of the HER-2 protein is related to malignant transformation in humans. Overexpression of the HER-2 protein is also intimately associated with the aggressiveness of the malignancy, being found in one-fourth of all invasive breast cancers. Overexpression of HER-2 protein is correlated with a poor prognosis in both breast and ovarian cancer.
In recent studies, antibodies directed against the extracellular binding domain (ECD) of HER-2 have been shown to confer inhibitory effects on tumor growth in vitro and in animal models (Hudziak, R. M., et al., Mol. Cell. Biol., 9:11–65–72, 1989; Tagliabue, E., et al., Int. J. Cancer 47:933–7, 1991; Drebin, J. A., et al., Proc. Natil. Acad. Scie. USA 83:9129–33, 1986; Drebin, J. A., et al., Oncogene, 2:273–7, 1988; Drebin, J. A., et al., Oncogene, 2:387–94, 1988; and Katsumata, M., et al., Nat. Med. 1:644–8. 1995.) In addition, Phase II and III clinical trials of a recombinant humanized anti-HER-2 monoclonal antibody, Trastuzumab, in patients with metastatic, HER-2-overexpressing breast cancers produced an overall response rate of 15% as a single agent. Trastuzumab has also been shown to improve survival when combined with cytotoxic chemotherapeutics (Baselga, J., et al., J. Clin. Oncol. 14:737–44, 1996; Pegram, M. D., et al., J. Clin. Oncol., 16:2659–71, 1988.). A number of vaccine approaches targeting a recombinant HER-2 protein, the HER-2 ECD, or the ECD of rat neu, which is the rat homolog of HER-2 have also been evaluated. For example, strain NFS mice immunized with a vaccinia virus recombinant that expresses the ECD rat neu developed a protective antibody response against subsequent challenge with neu-transformed NIH 3T3 cells (Bemards, R., et al., Proc. Natl. Acad. Sci. USA, 84:6854–8, 1987.). Immunization of BDIX rats with the same immunogen, however, did not result in antibody response nor did it inhibit the growth of syngeneic neu-expressing B 104 neuroblastoma cells, suggesting that this strategy was insufficient to induce immune responses in the rat. A polysaccharide-oncoprotein complex vaccine, consisting of the 147 amino-terminal amino acids of HER-2 ECD complexed with cholesteryl group-bearing mannan and pullulan, induced cellular and humoral immune responses that mediated rejection of HER-2-expressing sarcomas in BALB/c mice (Gu, X. G., et al., Cancer Res., 58: 3385–90, 1998.). Partial protection was shown in rat neu transgenic mice destined to develop mammary tumors by immunizing with either a purified rat neu ECD (Esserman, L. J., Cancer Immunol. Immunother., 47:337–42, 1999.) or neu-transfected allogeneic mouse fibroblasts (Cefai, D., et al., Int. J. Cancer, 83:393–400, 1999.)
Despite the results of the studies described above, it is still uncertain whether effective immune responses can be generated in humans using cell-or protein-based vaccine strategies targeting HER-2 or the HER-2 ECD, as HER-2 is a non-mutated, “self” antigen. Accordingly, it is desirable to have additional immunotherapeutic approaches for treating or preventing breast cancer and other malignancies with which overexpression of the HER-2 protein is associated.